Automatic or semi-automatic firearms typically include an auto loader that cycles a bolt carrier and bolt backward and forward after the firearm is fired. Depending on the particular firearm, the auto loader may be propelled by the recoil of the firearm and/or by the expanding gas associated with the discharge of a round. Rearward movement of the bolt carrier and bolt causes an extractor to engage and draw a spent round from the firing chamber. The bolt returns forward, often under action of a spring, after the round is ejected from the firearm. Forward movement of the bolt engages a fresh round from a magazine and pushes the round into the firing chamber for subsequent firing.
Automatic or semi-automatic firearms that utilize expanding combustion gas to power an auto loader may draw combustion gas from the firearm barrel after a round is fired. The pressurized combustion gas urges a piston that, in turn, moves the bolt of the firearm rearward to unlock the breach of the firearm, extract a round from the firing chamber and eject the spent round from the firearm. A spring typically urges the bolt forward to feed a fresh round into the firing chamber and to lock the firing chamber, completing the firing cycle of the firearm.